When to use a seperate DHCP server vs having router do DHCP?

Moon Child · August 2013

I have been studying for the Net+ exam which I hope to pass soon, but I still am a little confused about DHCP servers. For example if I chose to have all DHCP done by a seperate DHCP server rather than by my router why would I do that and what impact or problems could that cause? What would be some disadvantages to this setup and how would I troubleshoot DHCP errors if instead of the router acting as a DHCP I had an actual seperate physical server that did all the DHCP? What would happen to the IP addresses of the machines on the network when I switched over from a router to a DHCP server? Lets say I had a router that did DHCP and assigned all my computers IP addresses then one day out of the blue I decide hey I no longer want my router to do the DHCP instead I will give that job to a new DHCP server that I put in. What possible problems could arise out of this setup?

Darril · August 2013

You have quite a few questions there, and they are really beyond the scope of Network+, but here are some thoughts.

In general, you'd use a DHCP server if the extra load on the router loads it down. SOHO networks can get away with using just the DHCP service on a router. Large networks need one or more DHCP servers which relieves the router(s) of the additional work. As a general rule, if you have a domain, you'd typically have a DHCP server. Of course, this doesn't have to be the same server. A single server in a small business can be the domain controller (hosting Active Directory), a DNS server, and a DHCP server. A second server could be added with the same services for redundancy.

Network administrators wouldn't add a new DHCP server "out of the blue." Instead, once a need was identified, they would plan a migration from the router/DHCP to a server/DHCP.

If you don't plan the migration from a router/DHCP to a server/DHCP, it can result in having duplicate IP addresses assigned. Many DHCP clients will typically check for duplicate IPs and reject a duplicate IP address lease offer, which can slow down the time it takes for a system to get a lease.

One possible migration plan is:
Identify current lease length (let's say 7 days).
On router, reduce lease (to something like 4 hours) and limit scope to minimum requirements.
On server, create larger scope and exclude IPs from router's scope.
Wait until original lease length has expired (all clients will now either have IP from server or short lease from router)
Disable DHCP on router
Wait until new lease length on router expires (4 hours)
Remove exclusions from DHCP server

Hope this helps.

SteveO86 · August 2013

Yes, well beyond the N+. However DHCP can be done on the router or on the server. It all depends on the requirements and environment. If this is an office with a thousand users you will probably want your DHCP role to have its own physical server. If this is a small remote office you can not justify placing a entire server out there, you can consolidate the DHCP role and place it on the router. The newer ISR G2's from Cisco offer a considerable performance improvement compared the previous ISR G1 models.

For more information on DHCP on Cisco IOS 15 G2 routers IP Addressing: DHCP Configuration Guide, Cisco IOS Release 15M&T - Configuring the Cisco IOS DHCP Server* [Support] - Cisco Systems

As far as the advantages it's give and take, do you prefer Cisco CLI or Windows GUI?

In regards to moving a DHCP server, keep in mind a DHCP Database is kept on the DHCP server to keep track of the leases, and also that clients own the IP Address it was leased until the lease expires. So if you create a new DHCP server, it will most likely have a blank database and it may try to give out an address that is already in use. Most DHCP servers have a mechanism to check for 'in use' address however.

DDWingate · August 2013

Many enterprise routers don't have handy features like DHCP (since they're only focused on routing and doing a darn good job of it), so you'd want to use a SOHO router, which is a small router packed full of extras like DHCP. Of course, big businesses won't use SOHO's, they'll be using enterprise, so they'd have a separate server doing DHCP.

Router DHCP = small offices, homes and businesses
DHCP Server = enterprise, large businesses

Hondabuff · August 2013

If your company is big enough to use Active Directory then you should be using a DHCP Server.

When to use a seperate DHCP server vs having router do DHCP?

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